I took DS1 to see cars 2 as he loves the first one on dvd. He was just over 2.5. He wasn't scared at all but did get very restless. The movie was long and boring and I couldn't wait for it to finish, as about half way through he kept getting off the seat and trying to run away! I plan to wait a while before taking him again and next time I'll make sure it's a shorter kids movie. If your MIL was prepared to walk out early, then I'd say yes try it but otherwise, I'd wait.